The Voyager space probe, launched in 1977 and at this moment happily traveling outside our solar system, gets its electrical power via the heath caused by the decay of plutonium.
Since the half-life of plutonium, and the elements it decays into, is known, "aliens" can use it determine the age of the space craft.
At least that was until today, when I read the following article: Evidence for Correlations Between Nuclear Decay Rates and Earth-Sun Distance:
Unexplained periodic fluctuations in the decay rates of 32 Si and 226 Ra have been reported by groups at Brookhaven National Laboratory (32 Si), and at the Physikalisch-TechnischeBundesandstalt in Germany (226 Ra). We show from an analysis of the raw data in these experiments that the observed fluctuations are strongly correlated in time, not only with each other, but also with the distance between the Earth and the Sun. Some implications of these results are also discussed, including the suggestion that discrepancies in published half-life determinations for these and other nuclides may be attributable in part to differences in solar activity during the course of the various experiments, or to seasonal variations in fundamental constants.
If this is true (for real, the reality, ...) then the "aliens" will have a hard time figuring out the age of Voyager!
In addition to repeating long-term decay measurements on Earth, measurements on radioactive samp les carried aboard spacecraft to other planets would be very useful since the sa mple-Sun distance would then vary over a much wider range.Not limited to any knowledge in this field (besides the principles about decay of elements and their halflife, taught at science classes at high school), this is very intriguing stuff!